[bugfix,release] Fix alberta test if directory is passed via dunecontrol.

FindAlberta.cmake used ALBERTA_DIR for the root directory of the Alberta
installation while dunecontrol passed the directory in the variable
ALBERTA_ROOT. Therefore Alberta was not found in non-standard locations.
This patch fixes this by using ALBERTA_ROOT in FindAlberta.cmake now.
(cherry picked from commit 98f96cd60fc545fc27f3e76728c7fe84249e9b14)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>